Light-guide plate, area light source device, display device, manufacturing method for light guide plate

ABSTRACT

Provided are a light guide plate, an area light source device, a display device, and manufacturing method for the light guide plate such that the occurrence of uneven luminance is suppressed. The light guide plate ( 12 ) is characterized in that the light guide plate has a light entrance surface ( 12   a ) through which light enters, a light exit surface ( 12   c ) intersecting with the light entrance surface ( 12   a ) and through which light is output, and an opposite surface ( 12   b ) facing the light entrance surface ( 12   a ), wherein the light entering through the light entrance surface ( 12   a ) is guided to the opposite surface ( 12   b ) side and output from the light exit surface ( 12   c ), and the refractive index Nx in a direction perpendicular to the light entrance surface ( 12   a ) is higher than the refractive index Ny in a direction parallel to the light exit surface ( 12   c ) and parallel to the light entrance surface ( 12   a ).

TECHNICAL FIELD

The present invention relates to a light guide plate, a surface lightsource device, a display device, and a manufacturing method for thelight guide plate.

BACKGROUND ART

There has been conventionally known a display device in which a displayportion such as an LCD (Liquid Crystal Display) panel is illuminated bya surface light source device and an image is displayed thereon. Surfacelight source devices are roughly classified into a directly-below typein which a light source is disposed directly below an optical membersuch as various optical sheets, and an edge-light type in which thelight source is disposed on a side of the optical member. Of these, inthe edge-light type surface light source device, the light source isdisposed on the side of the optical member such as a light guide plate,and therefore it has an advantage that the surface light source devicecan be further thinned as compared with the directly-below type one, andhas been widely used in recent years.

In general, in the edge-light type surface light source device, thelight source is disposed at a position facing a light entrance surfacewhich is a side surface of the light guide plate, the light emitted fromthe light source enters the light guide plate through the light entrancesurface, and travels in a direction (light guide direction) orthogonalto the light entrance surface from the light entrance surface toward thefacing surface facing the light entrance surface while repeating thereflection by the light exit surface and the back surface facing thelight exit surface. The traveling direction of the light is changed by aprojection and recess shape and the like provided in the light guideplate, and therefore the light is emitted little by little from eachposition along the light guide direction of the light exit surfacetoward the LCD panel side (for example, Patent Document 1). Althoughsome of such light guide plates may be manufactured by an extrusionmethod by which a resin is formed by extrusion, in such a case, leakageof light that has entered through the light entrance surface occurs, andemission lines are seen in the light exit surface, that is, luminanceunevenness occurs in some cases.

Patent Document 1: Japanese Unexamined Patent Application, PublicationNo. 2009-217283

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

An object of the present invention is to provide a light guide plate, asurface light source device, a display device, and a manufacturingmethod for the light guide plate which are capable of suppressingoccurrence of luminance unevenness.

Means for Solving the Problems

The present invention solves the problem using the following solutionmeans. Even though a description will be given by assigning referencenumerals corresponding to embodiments of the present invention for easyunderstanding, the present invention is not limited thereto. Inaddition, a configuration described by assigning a reference numeralthereto may be modified as appropriate, and at least a part thereof maybe replaced with another component. A first invention provides a lightguide plate (12) comprising a light entrance surface (12 a) throughwhich light enters, a light exit surface (12 c) that intersects with thelight entrance surface and from which the light is emitted, and a facingsurface (12 b) that faces the light entrance surface, wherein the lighthaving entered through the light entrance surface is guided to thefacing surface side, and is emitted from the light exit surface, and arefractive index Nx in a direction perpendicular to the light entrancesurface is higher than a refractive index Ny in a direction parallel tothe light exit surface and parallel to the light entrance surface. Asecond invention provides a surface light source device (10) comprisingthe light guide plate (12) of the first invention, and a light sourceportion (11) that is provided at a position facing the light entrancesurface (12 a) of the light guide plate and emits the light to the lightentrance surface. A third invention provides a display device (1)comprising the surface light source device (10) of the second invention,and a display portion (2) that is disposed on a side of the light exitsurface (12 c) of the light guide plate (12) provided in the surfacelight source device. A fourth invention provides the display device (1)of the third invention, wherein the display portion (2) is a reflectiontype display portion. A fifth invention provides a manufacturing methodfor a light guide plate of manufacturing the light guide plate (12) ofthe first invention by an extrusion method, the method comprising aresin extrusion step of extruding a resin forming the light guide plateinto a forming plate (70) having a projection and recess shaped portion(73) corresponding to a shape of the light guide plate, and a lightguide plate forming step of pressing the forming plate while conveyingthe resin extruded in the resin extrusion step and forming the lightguide plate, wherein the forming plate is disposed so that a surface (73a) of the projection and recess shaped portion, the surfacecorresponding to the light entrance surface (12 a), is orthogonal to aconveying direction (S) of the resin.

Effects of the Invention

According to the present invention, the occurrence of luminanceunevenness can be suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a display device 1.

FIG. 2A and FIG. 2B are diagrams illustrating a light guide plate 12.

FIG. 3 is a diagram illustrating a shaped sheet used for manufacturingthe light guide plate 12 in detail.

FIG. 4 is a diagram illustrating a manufacturing method for the lightguide plate 12.

FIG. 5A to FIG. 5D are photographs in Example and Comparative Example,each showing, in a light exit surface, a light emitting state of lightthat has entered the light guide plate.

PREFERRED MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings. However, the technical scope of the presentinvention is not limited to the present embodiment. Note that in thedrawings used for the description of the embodiments, a size, ratio orthe like of each member may be changed or exaggerated as necessary.Hereinafter, embodiments of the present invention will be described withreference to the drawings and the like. Note that each drawing shownbelow, including FIG. 1, is a schematic view, and the size and shape ofeach portion are appropriately exaggerated for easy understanding. Inthis specification, words such as plate and sheet are used, but thesewords are generally used so that plate, sheet, and film are used in theorder of larger thickness, and also in this specification, the words areused in this manner. However, since such use does not have technicalsignificance, the words may be appropriately substituted with oneanother.

Numerical values of dimensions and the like and a material name and thelike of each member described herein are mere examples as embodiments,and are not limited to those shown below, and therefore they may beappropriately selected and used. In this specification, it is assumedthat terms which specify shapes and geometric conditions, for example,terms such as parallel and orthogonal also include a state having asimilar optical function and having an error that can be regarded asparallel and orthogonal in addition to strict meaning. In thisspecification, it is assumed that a sheet surface (plate surface, filmsurface) in each sheet (plate, film) represents a surface in a planedirection of the sheet (plate, film) when viewed as a whole.

Embodiments

FIG. 1 is a diagram illustrating a display device 1 of the presentembodiment. FIG. 2A and FIG. 2B are diagrams illustrating a light guideplate 12 of the present embodiment. FIG. 2A is a plan view of the lightguide plate 12, when viewed from a side of a light exit surface 12 c,and FIG. 2B is a partial cross-sectional view of an area b of FIG. 2A.Note that in the drawings shown below including FIG. 1 and the followingdescription, in a use state of the display device 1, of two directionsparallel to a screen of the display device 1 and orthogonal to eachother, a direction orthogonal to a light entrance surface 12 a of thelight guide plate 12 is referred to as an X direction, and a directionorthogonal to the X direction is referred to as a Y direction, for easyunderstanding. In addition, a direction (thickness direction) orthogonalto the screen of the display device 1 is referred to as a Z direction.Note that a +Z side of the thickness direction (Z direction) is referredto as an observer side, and a −Z side thereof is referred to as a backside.

The display device 1 of the present embodiment includes an LCD panel 2,and a surface light source device 10. The display device 1 illuminatesthe LCD panel 2 with the surface light source device 10 from theobserver side, and displays image information formed on the LCD panel 2.The screen of the display device 1 of the present embodiment correspondsto a surface (hereinafter, referred to as a “display surface”) 10 aclosest to the +Z side (observer side) in the surface light sourcedevice 10, and a “front direction” of the display device 1 represents anormal direction relative to the display surface 10 a, is parallel tothe Z direction, and coincides with the normal direction relative to aplate surface of the light guide plate 12 (described later), and thelike.

The LCD panel 2 is a reflection type display portion that is formed froma liquid crystal display element and forms the image information on thedisplay surface thereof. The LCD panel 2 is formed in a substantiallyflat plate shape. An external shape of the LCD panel 2 is formed in arectangular shape when viewed from the Z direction, and has two oppositeedges parallel to the X direction and two opposite edges parallel to theY direction. The surface light source device 10 is a device forilluminating the LCD panel 2 from the observer side (+Z side), andincludes a light source portion 11, the light guide plate 12, and thelike. The surface light source device 10 is an edge light type surfacelight source device (front light).

The light source portion 11 is a portion that emits light forilluminating the LCD panel 2. The light source portion 11 is disposedalong the Y direction at a position facing the light entrance surface 12a (described later) which is one end surface of the light guide plate 12(on the +X side) in the X direction. The light source portion 11 isformed to have a plurality of point light sources arranged at apredetermined interval in the Y direction. For the point light source,an LED (Light Emitting Diode) light source is used. Note that the lightsource portion 11 may be, for example, a linear light source such as acold-cathode tube, or may be in a form of a light source disposed on anend surface of a light guide extending in the Y direction. In addition,a reflection plate (not illustrated) may be provided to cover theoutside of the light source portion 11, from the viewpoint of improvingthe use efficiency of light emitted by the light source portion 11.

The light guide plate 12 included in the surface light source device 10is formed in a rectangular shape when viewed from a front direction (Zdirection), and has two opposite edges parallel to the X direction, andtwo opposite edges parallel to the Y direction. The light guide plate 12is a substantially flat plate-shaped member that guides the light, inwhich the light emitted from the light source portion 11 enters thelight entrance surface 12 a, is guided in the light guide plate 12, andthen is emitted from the light exit surface 12 c. In the presentembodiment, the light entrance surface 12 a and a facing surface 12 bfacing each other are located at both end portions (an end portion onthe +X side and an end portion on the −X side) of the light guide plate12 in the X direction, respectively, and correspond to two edgesextending parallel to the Y direction when viewed in the normaldirection (Z direction) relative to the plate surface. In addition, thelight exit surface 12 c and a back surface 12 d facing each other arelocated at both end portions (an end portion on the −Z side and an endportion on the +Z side) of the light guide plate 12 in the Z direction,respectively. The plate surface of the light guide plate 12 is parallelto an XY surface, and the back surface 12 d as the display surface 10 aof the display device 1 is a surface parallel to the plate surface. Afirst side surface 12 e and a second side surface 12 f facing each otherare located at both end portions (an end portion on the −Y side and anend portion on the +Y side) of the light guide plate 12 in the Ydirection, and correspond to two edges extending parallel to the Xdirection when viewed in the normal direction (Z direction) relative tothe plate surface.

The light guide plate 12 allows the light emitted from the light sourceportion 11 to enter through the light entrance surface 12 a, travel to afacing surface 12 b side (X2 side) facing the light entrance surface 12a while allowing it to be totally reflected by the light exit surface 12c and the back surface 12 d, and then be appropriately emitted from thelight exit surface 12 c to an LCD panel 2 side (−Z side) while mainlyguiding the light in the X direction. The light guide plate 12 of thepresent embodiment is formed in a rectangular shape when viewed from thethickness direction (Z direction), and among surfaces parallel to thethickness direction, the surfaces parallel to the longitudinal direction(Y direction) are referred to as the light entrance surface 12 a and thefacing surface 12 b, from the viewpoint of guiding more light.

In the present embodiment, as illustrated in FIG. 2A and FIG. 2B, aprojection and recess portion 13 is formed on the light exit surface 12c of the light guide plate 12, the projection and recess portion 13being formed by fine projection and recess features that are alternatelyformed in a direction (X direction) perpendicular to the light entrancesurface 12 a. This enables the light guided in the light guide plate 12to be emitted from the light exit surface 12 c as appropriate. Asillustrated in FIG. 2B, the projection and recess portion 13 extends inthe Y direction of the light guide plate 12, and is comprised ofprojection portions 13 a each having a rectangular cross section andrecess portions 13 b each having a rectangular cross section, theprojection portions 13 a and the recess portions 13 b being alternatelyformed in the X direction. The projections and recesses forming theprojection and recess portion 13 are formed to be fine in size, and, forexample, the depth of the recess portion 13 b is about 0.1 μm or moreand 3.0 μm or less. In addition, each of the bottom width of the recessportion 13 b and the top width of the projection portion 13 a is about 1μm or more and 30 μm or less.

Note that the shape of the projection and recess portion 13 is notlimited to the above-described example, and another form may be appliedor a plurality of forms may be combined. Examples of a projection andrecess portion of another form include a form of having a predeterminedcross section to extend to be sloped with respect to the width direction(Y direction) of the light guide plate 12, a form of having atwo-dimensional shape in which a plurality of conical projectionportions are arranged in a matrix configuration on the plan view, and aform in which a plurality of projection portions are arranged in adot-like manner on the plan view.

The light guide plate 12 can be formed of various materials. Examplesthereof include thermoplastic resins such as a polymer resin having analicyclic structure, a methacrylate resin, a polycarbonate resin, apolystyrene resin, an acrylonitrile-styrene copolymer, a methylmethacrylate-styrene copolymer, an ABS resin, and polyether sulfone, andan epoxy acrylate or urethane acrylate reactive resin. These materialsare widely used as materials for an optical member such as a light guideplate, have excellent mechanical characteristics, opticalcharacteristics, stability, processability, and the like, and areinexpensively available.

Here, the light guide plate 12 of the present embodiment is formed sothat the refractive index Nx in the direction (X direction or adirection from the light entrance surface 12 a toward the facing surface12 b) perpendicular to the light entrance surface 12 a is higher thanthe refractive index Ny in the width direction (Y direction or adirection from the first side surface 12 e toward the second sidesurface 12 f) parallel to the light exit surface 12 c and parallel tothe light entrance surface 12 a (Nx>Ny). In this way, the light guideplate 12 of the present embodiment can emit uniformly, from the lightexit surface, the light that enters through the light entrance surface12 a and is guided in the light guide plate 12. If the refractive indexNx in the direction perpendicular to the light entrance surface 12 a islower than the refractive index Ny in the width direction (Nx<Ny), thelight that has entered through the light entrance surface may leak, andin this case, emission lines are seen in the light exit surface andluminance unevenness occurs, which is not desirable.

It is believed that when the refractive index (Nx) in the X direction(light guide direction) perpendicular to the light entrance surface islower than the refractive index (Ny) in the Y direction (Nx<Ny), thelight that has entered through the light incident surface tends totravel in the light guide direction (X direction), and converges toomuch in the X direction, resulting in being easily seen as the emissionlines. On the other hand, it is believed that when the refractive indexin the X direction (light guide direction) is higher than the refractiveindex in the Y direction (Nx>Ny), a part of the light that has enteredthrough the light incident surface is dispersed and travels in adirection intersecting with the X direction, thereby preventing thelight from converging too much in the X direction, which suppresses theoccurrence of the emission lines. The light guide plate 12 of thepresent embodiment is formed of a polycarbonate resin having opticalanisotropy, and the refractive index Nx in a direction perpendicular tothe light entrance surface 12 a is 1.59, and the refractive index Ny ina direction perpendicular to the first side surface 12 e is 1.58.

Here, the light guide plate 12 is particularly effective when being usedas a front light as in the display device 1 of the present embodiment.In the light guide plate used for a back light, since not only an LCDpanel but also a diffusion plate, a prism sheet, and the like arearranged on the light exit surface of the light guide plate, even whenthe above-described emission lines are generated on the light exitsurface of the light guide plate, the emission lines are hardlyconspicuous by the diffusion plate and the like in some cases. Incontrast, in the light guide plate 12 used for a front light, the backsurface 12 d of the light guide plate serves as the display surface 10 aof the display device 1, and therefore a possibility that the emissionlines are seen is extremely high when the emission lines are generated.Accordingly, when the light guide plate 12 satisfying Nx>Ny is providedin the surface light source device 10 as in the display device 1 of thepresent embodiment, the above-described emission lines can be preventedfrom being seen.

Since the display device 1 of the present embodiment includes thereflection type LCD panel 2 as described above, when an amount ofincident light such as external light into the LCD panel 2 issufficient, the light source portion 11 provided in the surface lightsource device 10 is used in an off state. On the other hand, when theamount of incident light into the LCD panel 2 is insufficient andtherefore an image displayed on the display surface 10 a is dark, thedisplay device 1 is used in a state where the light source portion 11emits light. Therefore, when the display device 1 is designed to bedisposed at a relatively light place, a time period for which the lightsource portion 11 is lighted is considerably shorter, and therefore, thenumber of point light sources used in the light source portion 11 isreduced, in some cases, from the viewpoint of reducing the cost andreducing the power consumption of the display device 1. In such a case,when the refractive index in the light guide direction (X direction) islower than that in the Y direction (Nx<Ny), the interval between thepoint light sources becomes wider, and the emission lines may be seenmore prominently. However, as in the present embodiment, the refractiveindex in the light guide direction (X direction) is made higher than therefractive index in the Y direction (Nx>Ny), whereby the emission linescan be prevented from being seen even in such a case.

It is preferable that the thickness of the light guide plate 12 (adistance from the light exit surface 12 c (a top portion of a projectionportion) to a back surface 12 d, in the thickness direction of the lightguide plate) is 0.1 mm to 1.0 mm. If the thickness of the light guideplate 12 is 0.1 mm or less, the number of times of reflection of lightbeing guided in the light guide plate becomes too large. If thethickness of the light guide plate 12 is 1.0 mm or more, the number oftimes of reflection of light being guided in the light guide platebecomes too small. Therefore, both cases are not preferable because itis difficult to uniformly emit the light from the light exit surface ofthe light guide plate. Note that the thickness of the light guide plate12 of the present embodiment is, for example, 0.4 mm. A refractive indexdifference between the refractive index Nx and the refractive index Nyof the light guide plate 12 (Nx−Ny) is preferably 0.002 or more, morepreferably 0.005 or more, and further preferably 0.01 or more, from theviewpoint of effectively preventing the above-described emission linesfrom being seen. Note that, since the refractive index difference variesdepending on the thickness of the light guide plate, more specifically,tends to be reduced when the thickness of the light guide plate isreduced, it is preferable that the refractive index difference is 0.002or more when the thickness of the light guide plate 12 is 0.1 mm whichis a lower limit value of the above-described preferable range.

Manufacturing Method for Light Guide Plate 12

Here, before describing a manufacturing method for the light guide plate12, a shaped sheet 70 used for forming the light guide plate 12 will bedescribed. FIG. 3 is a diagram illustrating the shaped sheet 70 used forforming the light guide plate 12. As illustrated in FIG. 3, the shapedsheet 70 is a long forming plate made of a resin having flexibility, inwhich a forming layer 72 is stacked over a base material portion 71, andis wound into a roll before manufacturing the light guide plate 12. Thebase material portion 71 is a base material serving as a base of theshaped sheet 70, and is formed of, for example, a polyethyleneterephthalate (PET) resin. Note that, in addition to the PET resin,transparent resins such as a polycarbonate (PC) resin, a methylmethacrylate-butadiene-styrene (MBS) resin, a methylmethacrylate-styrene (MS) resin, an acrylic-styrene (AS) resin, and anacrylonitrile-butadiene-styrene (ABS) resin can be also used. Theforming layer 72 is a layer in which a projection and recess shapedportion 73 corresponding to the projection and recess portion 13 of thelight guide plate 12 is formed on a surface (front surface) opposite tothe base material portion 71 side. The forming layer 72 is formed of,for example, a urethane acrylate ultraviolet curable resin. Note that,in addition to the urethane acrylate ultraviolet curable resin,ultraviolet curable resins such as polyester acrylate, epoxy acrylate,polyether acrylate, polythiol, and butadiene acrylate can be also used.In addition to the above-described ultraviolet curable resin, anelectron beam-curable resin can be used as an energy ray-curable resin.A plurality of projection and recess shaped portions 73 to be providedon the forming layer 72 are arranged on the front surface of the forminglayer 72, to simultaneously form a plurality of light guide plates 12.In the present embodiment, the plurality of projection and recess shapedportions 73 are formed in three rows in a direction parallel to a shortedge of the long shaped sheet 70 and in a plurality of stages in adirection parallel to a long edge thereof.

Next, a method of manufacturing the light guide plate 12 by extrusionwill be described. FIG. 4 is a diagram illustrating the manufacturingmethod for the light guide plate 12. First, as illustrated in FIG. 4,the shaped sheet 70 wound into a roll is wound off, and is successivelyconveyed between a first roll 81 and a second roll 82 disposed toprovide a predetermined space with respect to the first roll 81. Here, aconveying direction S of the shaped sheet 70 is a direction parallel toa direction perpendicular to the short edge of the shaped sheet 70. Amelted thermoplastic resin composition 85 (resin forming the light guideplate) is extruded between a surface of the shaped sheet 70 on a side inwhich the projection and recess shaped portions 73 are formed and thesecond roll 82, from a nozzle 86 (resin extrusion step).

Next, the extruded thermoplastic resin composition 85 is sandwiched andpressed between the first roll 81 and the second roll 82 while beingconveyed together with the shaped sheet 70, to form the light guideplates 12 (light guide plate forming step). In this way, thethermoplastic resin composition 85 is filled into the projection andrecess shaped portions 73 provided on the front surface of the forminglayer 72 (see FIG. 3) of the shaped sheet 70, and is hardened by coolingwith the second roll 82 and ambient air (outside air, an operatingenvironment of a manufacturing device, and the like), thereby beingformed in a shape along the projection and recess shaped portion 73 ofthe shaped sheet 70. Then, the thermoplastic resin composition 85 isfurther cooled through a third roll 83 and a fourth roll 84, so that thefinal shape is fixed. In this way, a sheet 10′ with multiple light guideplates can be obtained in which multiple light guide plates 12 arearranged in a matrix configuration. Next, a peeling roll 87 is used toseparate the sheet 10′ with multiple light guide plates from the shapedsheet 70. Finally, the sheet 10′ with multiple light guide plates ispunched, cut or the like, so that a plurality of light guide plates 12(see FIG. 2A) can be individually obtained.

Therefore, as a result of an exhaustive study by the inventors of thepresent invention, it has been found that since the thermoplastic resincomposition 85 extruded from the nozzle 86 extends in the conveyingdirection S, the refractive index in the conveying direction S tends tobe higher than the refractive index in the direction perpendicular tothe conveying direction S. Thus, in the present embodiment, asillustrated in FIG. 3, each of the projection and recess shaped portions73 provided on the shaped sheet 70 is formed so that a surface 73 acorresponding to the light entrance surface 12 a of the light guideplate 12 (a surface parallel to the longitudinal direction of the lightguide plate 12) is parallel to the short edge w1 of the shaped sheet 70.That is, each of the projection and recess shaped portions 73 providedon the shaped sheet 70 is arranged on the forming layer 72 so that thesurface 73 a corresponding to the light entrance surface 12 a of thelight guide plate 12 is orthogonal to the conveying direction S of theshaped sheet 70.

Therefore, the thermoplastic resin composition 85 extruded from thenozzle 86 extends in the direction perpendicular to the surface 73 a ofthe projection and recess shaped portion 73, the surface 73 a serving asthe light entrance surface 12 a of the light guide plate 12. In thisway, the produced sheet 10′ with multiple light guide plates (lightguide plates 12) is formed so that the refractive index Nx in thedirection (light guide direction) perpendicular to the light entrancesurface 12 a of the light guide plate 12 is higher than the refractiveindex Ny in the direction perpendicular to the first side surface 12 e.As described above, the light guide plate 12 satisfying the refractiveindex Nx> the refractive index Ny is manufactured. Note that therefractive index difference between the refractive index Nx and therefractive index Ny can be adjusted by appropriately adjusting the resintemperature and cooling temperature of the thermoplastic resincomposition 85 extruded from the nozzle 86, the conveying speed of theshaped sheet 70, tension in the conveying direction S, and the like.

Since the light guide plate was conventionally manufactured by aninjection molding or the like, as disclosed in, for example, JapaneseUnexamined Patent Application, Publication No. Hei10-142601, the lightguide plate satisfying Nx>Ny could not be manufactured like the lightguide plate 12 of the present embodiment. It is also possible tomanufacture the light guide plate 12 satisfying Nx>Ny by forming theprojection and recess portion 13 in one surface of a resin film havingoptical anisotropy, but in this case, it may be very difficult to form adesired projection and recess portion with high accuracy, which maycause an increase in the manufacturing cost. In contrast, as describedabove, the shaped sheet 70 and the extrusion method are used, wherebythe light guide plate 12 satisfying Nx>Ny can be manufactured moreeasily and less expensively.

Evaluation of Luminance Unevenness

Next, there will be described evaluation results of relationshipsbetween the refractive index in the direction perpendicular to the lightentrance surface of the light guide plate and the refractive index inthe direction perpendicular to each side surface, and the occurrence ofluminance unevenness in the light exit surface of the light guide plate.FIG. 5A to FIG. 5D are photographs in Example and Comparative Example,each showing, in the light exit surface, a light emitting state of thelight that has entered the light guide plate. FIG. 5A is a photographwhich is taken from a direction perpendicular to the light exit surfaceof the light guide plate of Example, and FIG. 5B is a photograph whichis taken from a direction inclined by 45 degrees with respect to thelight exit surface of the light guide plate of Example. FIG. 5C is aphotograph which is taken from a direction perpendicular to the lightexit surface of the light guide plate of Comparative Example, and FIG.5D is a photograph which is taken from a direction inclined by 45degrees with respect to the light exit surface of the light guide plateof Comparative Example.

The light guide plate of Example used for the evaluation is formed in amanner similar to the light guide plate 12 of the above-describedembodiment, and is formed of a polycarbonate resin. In the light guideplate of Example, the refractive index Nx in the direction (light guidedirection) perpendicular to the light entrance surface is set to 1.59,and the refractive index Ny in the direction (direction perpendicular toeach side surface) from the first side surface toward the second sidesurface is set to 1.58, so that the refractive index Nx is higher thanthe refractive index Ny (Nx>Ny). The light guide plate of ComparativeExample is formed of a polycarbonate resin. In light guide plate ofComparative Example, the refractive index Nx in the direction (lightguide direction) perpendicular to the light entrance surface is set to1.58, and the refractive index Ny in the direction (directionperpendicular to each side surface) from the first side surface towardthe second side surface is set to 1.59, so that the refractive index Nxis lower than the refractive index Ny (Nx<Ny).

Here, the refractive indexes Nx and Ny of each light guide plate weremeasured using KOBRA series measurement device (KOBRA-WR) made by OjiScientific Instruments Co., Ltd. As the light guide plates of Exampleand Comparative Example used for measurement of the refractive indexes,plate-like test samples in which no projection and recess portion isprovided on the light exit surface were used. In the above-describedmeasurement device, a light source portion, a polarizer, an analyzer,and a light-receiving portion were arranged in order, and a mountingstage for the test sample was provided between the polarizer and theanalyzer. Each test sample described above was mounted on the mountingstage, a single-wavelength beam was emitted from the light sourceportion, and a phase difference and an orientation angle of the testsample were measured from angular dependence of transmitted lightintensity when the test sample was rotated once around a beam axis whilethe polarizer and the analyzer are arranged in a parallel nicols state,thereby obtaining the refractive indexes in the X direction and the Ydirection. Here, a beam having a wavelength of 590 nm was used for thebeam emitted from the light source portion.

As shown in FIG. 5C and FIG. 5D, in the light guide plate of ComparativeExample, it was confirmed that leakage of the light that entered throughthe light entrance surface was seen as emission lines in the light exitsurface. In contrast, in the light guide plate of Example, as shown inFIG. 5A and FIG. 5B, it was confirmed that leakage of the light thatentered through the light entrance surface was suppressed as much aspossible, was significantly prevented from being seen as emission linesin the light exit surface, and the light was uniformly emitted.

As described above, the light guide plate 12 of the present embodimentis formed so that the refractive index Nx in the direction (directionperpendicular to the light entrance surface 12 a) from the lightentrance surface 12 a toward the facing surface 12 b is higher than therefractive index Ny in the direction (direction perpendicular to eachside surface) from the first side surface 12 e toward the second sidesurface 12 f (Nx>Ny). Thus, the light guide plate 12 can prevent theluminance unevenness from occurring in the light emitted from the lightexit surface 12 c, and can emit the guided light uniformly from thelight exit surface 12 c.

In the manufacturing method for the light guide plate 12 of the presentembodiment, the surface 73 a of the projection and recess shaped portion73 of the shaped sheet 70, the surface 73 a corresponding to the lightentrance surface 12 a of the light guide plate 12, is disposed to beorthogonal to the conveying direction S of the resin, whereby the lightguide plate 12 satisfying the refractive index Nx> the refractive indexNy can be manufactured easily.

Although embodiments of the present invention are described above, thepresent invention is not limited to the embodiments described above,and, as in variation forms which will be described later, variousvariations and modifications are possible, and they also fall within thetechnical scope of the present invention. The effects described in theembodiments are simply a list of the most preferred effects producedfrom the present invention, and there is no limitation to the effectsdescribed in the embodiments. Although the embodiments described aboveand the variation forms which will be described later can be used bybeing combined as necessary, the detailed description thereof will beomitted.

Variation Forms

(1) In the above-described embodiment, there is described an example inwhich the light guide plate 12 is used for the front light of thedisplay device, but the present invention is not limited thereto, andthe light guide plate 12 may be used for a back light of a transmissiontype display device. In this case, the LCD panel needs to be of thetransmission type that transmits the light from the back side. A lightcontrol sheet such as a prism sheet, or a light diffusion sheet may beprovided between the LCD panel and the light guide plate.

(2) In the above-described embodiment, there is described an example inwhich the light guide plate 12 is formed by the shaped sheet 70 that iswound off, but the present invention is not limited thereto. Forexample, the light guide plate 12 may be manufactured using a roll platein which a projection and recess shaped portion is formed on acylindrical peripheral side surface, instead of using the shaped sheet.

(3) In the above-described embodiment, there is described an example inwhich the display surface 10 a of the display device 1 serves as theback surface 12 d of the light guide plate 12, but the present inventionis not limited thereto, and a transparent protective sheet forpreventing scratching of the light guide plate may be attached to theback surface 12 d of the light guide plate 12, so that a front surfaceof the protective sheet serves as the display surface. In this case, itis desirable that the material having the refractive index lower thanthat of the material used for the light guide plate 12 is applied to ajoint layer for joining the back surface 12 d of the light guide plate12 and the protective sheet, from the viewpoint of efficiently totallyreflecting the light in the light guide plate 12. When the light guideplate 12 is formed of a polycarbonate resin as in the above-describedembodiment, it is desirable that for example, an acryl resin having therefractive index lower than that of the polycarbonate resin is used forthe joint layer.

(4) In addition, the light exit surface 12 c of the light guide plate 12and the LCD panel 2 may be joined by a transparent joining member. Atthis time, the joining member may be provided to fill the projection andrecess shape formed on the light exit surface 12 c of the light guideplate 12. Note that, also in this case, it is desirable that a materialhaving the refractive index lower than that of the material used for thelight guide plate 12 is used for the joining member, from the viewpointof efficiently totally reflecting the light in the light guide plate 12.

EXPLANATION OF REFERENCE NUMERALS

-   1 display device-   10 surface light source device-   11 light source portion-   12 light guide plate-   12 a light entrance surface-   13 projection and recess portion-   70 shaped sheet-   71 base material portion-   72 forming layer-   73 projection and recess shaped portion

The invention claimed is:
 1. A surface light source device, comprising:a light guide plate for use for front light, the light guide platehaving a thickness; and a light source portion that is provided at aposition facing a light entrance surface of the light guide plate andemits light to the light entrance surface, the light guide platecomprising: the light entrance surface which is parallel to a directionof the thickness of the light guide plate and through which the lightenters; a light exit surface which is perpendicular to the lightentrance surface and perpendicular to the direction of the thickness andfrom which the light is emitted; and a facing surface that faces thelight entrance surface, wherein the light having entered through thelight entrance surface is guided to the facing surface side, and isemitted from the light exit surface, wherein (i) a refractive index Nxof the light guide plate in an orientation perpendicular to the lightentrance surface and perpendicular to the thickness, and (ii) arefractive index Ny of the light guide plate in an orientation parallelto the light exit surface and parallel to the light entrance surface,and wherein the refractive index Nx is higher than the refractive indexNy.
 2. A display device, comprising: the surface light source deviceaccording to claim 1; and a display portion that is disposed on a sideof the light exit surface of the light guide plate provided in thesurface light source device.
 3. The surface light source deviceaccording to claim 1, wherein a projection and recess portion is formedon the light exit surface, the projection and recess portion extendingin a direction parallel to the light entrance surface and being formedby fine projection and recess features that are alternately formed in adirection perpendicular to the light entrance surface.
 4. The surfacelight source device according to claim 1, wherein the light guide platehas a thickness of 0.1 mm to 1.0 mm.
 5. The surface light source deviceaccording to claim 1, wherein a difference (Nx−Ny) between therefractive index Nx and the refractive index Ny of the light guide plateis 0.002 or more.
 6. The surface light source device according to claim1, wherein the light guide plate has a thickness of 0.1 mm to 1.0 mm,and a difference (Nx−Ny) between the refractive index Nx and therefractive index Ny of the light guide plate is 0.002 or more.
 7. Thedisplay device according to claim 2, wherein the display portion is areflection type display portion.
 8. The surface light source deviceaccording to claim 6, wherein the recess portion has a depth of about0.1 μm or more and 3.0 μm or less, and a bottom width of about 1 μm ormore and 30 μm or less, and the projection portion has a top width ofabout 1 μm or more and 30 μm or less.